1. Field of the Invention
The present invention relates generally to a pneumatic radial tire adopted to be used for heavy-duty load vehicles such as trucks, buses, etc. and more specifically to an improvement in wandering performance without degrading various other performances required for the tire.
2. Description of the Prior Art
As an undesired phenomenon produced in heavy-duty radial tires, "wandering" in particular, has been pointed out as a problem to be solved. This wandering phenomenon is: When a heavy-duty load vehicle such as trucks or buses travels along rut (wheel track or mark) formed on a busy paved road surface by wear or sinking due to vehicle weight, the driver loses control of the vehicle and therefore has difficulty in steering the vehicle when keeping the vehicle traveling straight along the slope of a rut or changing the travel line across a rut.
In general, when heavy-duty pneumatic tires travel on a road on which ruts are formed, the bias-structure tires are relatively well controllable by the driver, while the radial-structure tires are usually not considered as being controllable by the drivers. In more detail, when the radial tire comes on the top of a rut in changing the travel lane, a lateral force is immediately produced in such a way that the tire is pulled down to the bottom of the rut; as a result, the radial tire tends to run on the top of another adjacent rut or further beyond the top thereof, being subjected to a strong impulsive reactive force. Further, when the radial front tire travels on and along the slope of a rut, it is possible to keep tire travelling parallel to the slope thereof, only when the driver holds the steering wheel with a strong self-alignment effort. Therefore, there exists a strong demand for heavy-duty vehicle radial tires excellent in wandering prevention performance from both the manufacturers and the users.